Food containers like cups, which have to withstand temperatures around the boiling point of water, because they are used for hot drinks like tea, coffee or soup, are usually not manufactured of polylactides, although it is known that polylactides are biodegradable polymers which are decomposed by the action of microbes and enzymes into lactide acid, carbon dioxide and water.
One of the main reasons that polylactides are not suitable for the manufacturing of such cups is the low glass transition temperature of these polymers, which causes softening of the material already about 50° C., which is more than 40° C. below the application temperature of these polymers.
Another important disadvantage of polylactides are the crystallization properties of these polymers. Although homopolymers, PDLA respectively PLLA, are crystalline, the crystallization rate of these polymers is relatively slow and the polymers exhibit a behavior particularly similar to that of a non-crystalline resin.
In view of this relatively long time for crystallization, compositions containing PLA, even homopolymers, do not crystallize sufficiently during the usual short cycle times for the production of thermoformed articles, e.g. by thermoforming with moulding. Therefore, the use of these biodegradable polymers is rather disadvantageous.
Consequently, a considerable portion of food containers, especially cups for hot beverages and predominantly cups for a single use, are still manufactured of plastic resins produced from fossil sources, and not of plastic resin produced from renewable raw material, although the thermoformable PLA, PLLA and PDLA, are produced from renewable raw materials and therefore are biobased as well as biodegradable.
In order to be able to provide containers for hot beverages, especially cups for single use, manufactured of biobased and biodegradable polymers like PLLA or PLA, the above identified disadvantages of these polymers have to be overcome. Especially the need for the production of such containers, especially cups, in short cycle times has to be satisfied by increasing the rate of crystallization and the resistance against deformation at elevated temperatures, especially if such containers, especially cups, are used for hot beverages like hot coffee or tea.
Therefore, an object of the present invention was to provide thermoformable compositions containing as biobased polymers poly-L-lactide with less than 1 mol % of D-lactoyl-units (PLLA) or poly-L-lactide with from 1 mol % to 5 mol % of D-lactoyl-units (PLA), which guarantee a rate of crystallization allowing to usual short cycle times in the production of thermoformed food containers, especially cups, and the provision of such containers having excellent mechanical properties up to temperatures of 100° C.